A division that includes the defending World Series champion and both combatants in last season’s NLCS also features plenty of uncertainty.

That will happen when two of the game’s premier sluggers—Albert Pujols and Prince Fielder—leave via free agency. In fact, it can be argued that the Pittsburgh Pirates, who have endured 19 consecutive losing seasons, were the most stable of the division’s six franchises this winter.

The Milwaukee Brewers and St. Louis Cardinals must replace superstars, the Cincinnati Reds made a couple of risky trades, the Chicago Cubs’ revamped front office has begun a rebuilding phase and the Houston Astros have a new owner and one heck of a challenge ahead of them.

Brewers

Not only must Milwaukee replace Fielder for the entire season, it likely will play the first one-third of the season without left fielder Ryan Braun, whose appeal of a 50-game suspension is pending. Third baseman Aramis Ramirez, signed as a free agent, will provide some offensive relief, and the team will use the spring to decide whether Mat Gamel will be its everyday first baseman.

No one expects Gamel, a fourth-round draft pick in 2005, to be Fielder. However, the converted third baseman must improve on his .222 average in parts of four major league seasons. Gamel hit .310 with 28 homers and 96 RBIs at Class AAA last season, but that offensive prowess has yet to translate to the majors.

If Braun must miss 50 games, the Brewers likely will turn to fourth outfielder Carlos Gomez, though that would be quite a drop-off from the NL MVP.

Cardinals

All the defending World Series champions must do is adjust to life without Pujols, manager Tony La Russa and pitching coach Dave Duncan. However, things aren’t as dire as they appear. Newcomer Carlos Beltran will help make up for the loss of Pujols, and Adam Wainwright will provide another ace for the rotation after missing the 2011 season because of Tommy John surgery.

For a team that suffered so many offseason losses, St. Louis has few lineup questions. In fact, the biggest threat to their repeat quest might be injuries, as several regulars (Matt Holliday, Rafael Furcal and David Freese among them) have been injury-prone. The one area that must be settled is second base, where Skip Schumaker, Daniel Descalso and Tyler Greene will vie for playing time. Schumaker began the 2011 season as the starter but eventually shared the position with the now-departed Nick Punto and Ryan Theriot.

Reds

Cincinnati is in win-now mode, as general manager Walt Jocketty made clear with his aggressive offseason approach. He surrendered first base prospect Yonder Alonso to acquire righthander Mat Latos from the San Diego Padres, and he bolstered his bullpen by plucking lefthander Sean Marshall from the Cubs. Jocketty also landed closer Ryan Madson at a bargain price and added power-hitting Ryan Ludwick to battle for the left field job.

However, Jocketty recently downplayed the Reds’ interest in free-agent righthander Roy Oswalt for good reason: The team can’t afford him and doesn’t need him. With the arrival of Latos, Cincinnati has more starting pitchers than rotation spots. Latos, Johnny Cueto and Bronson Arroyo are locks, leaving Mike Leake, Homer Bailey, Aroldis Chapman and Jeff Francis to compete for two spots. All eyes will be on Chapman, who is attempting to return to a starting role after pitching exclusively as a reliever in the majors.

Pirates

After making a run at respectability in 2011—the team had at least a share of first place until late July—Pittsburgh loaded up on under-the-radar acquisitions over the winter. The Pirates will spend the spring discovering how many of them will stick.

Among the additions, all of whom are coming off subpar seasons: third baseman Casey McGehee, lefthander Erik Bedard, outfielder Nate McLouth and shortstop Clint Barmes. If nothing else, McGehee provides insurance at third and the competition there could spark Pedro Alvarez to finally reach his potential. The oft-injured Bedard would bring a much-needed veteran presence to the rotation, and Barmes could fill the void the team has had at short for several seasons. McLouth, who was a bust in his three seasons with the Atlanta Braves after his 26-homer season with the Pirates in 2008, could press young Alex Presley for the starting job in left.

Cubs

The focus this spring will be at the corner infield spots, where the biggest news will center on the moves Chicago didn’t make.

Many expected the Cubs to land Fielder, but new president of baseball operations Theo Epstein wasn’t bluffing when he repeatedly said the team wasn’t a major player for the free-agent slugger. Instead, Bryan LaHair, a 29-year-old with 195 major league at-bats will start at first base—unless Anthony Rizzo, acquired in a trade with the Padres, wows the team this spring.

For the first time since the start of the 2003 season, Ramirez, who led the team with 93 RBIs last season, won’t be manning third base. To fill that void, the Cubs dealt for Ian Stewart, who went homerless with a .159 average and six RBIs with the Colorado Rockies last season. Chicago is hoping for a return to Stewart’s 2009 production (25 homers, 70 RBIs).

Astros

There are roughly as many questions surrounding this team in its final season in the NL as it suffered losses last season. And there were 106 of those. Other than Carlos Lee, the starting lineup’s most recognizable name to even diehard fans probably is shortstop Jed Lowrie, acquired from the Boston Red Sox during the offseason.

Among the many questions this spring: Who’s on first? Actually, that might depend on who’s on third. Houston will experiment with Brett Wallace, last season’s opening-day first baseman but a third baseman earlier in his career, at the hot corner. Wallace will have to outlast Chris Johnson, who like Wallace has lost much of his previous luster, and Jimmy Paredes. Ideally, Wallace will win the third base job, opening first for Lee.